Pretty much a cluster**** to try and figure out exactly what happened for now. The plane should have been too high for MANPADS to shoot down (Conspiracy theory prediction: they'll use confusion about the altitude of MH370 to cloud this point here as well), but both the Ukraine and Russia have SA 11 Buk missile launchers that could handle that altitude, and Russia has modern versions that could have hit the plane from inside Russia. In addition to both governments, it's one of those weapons that Russia's been known to give out to friendly groups, and there's been all kinds of vehicles stolen from the Ukraine military (their security had fallen apart enough at one point that somebody took a tank for a joy ride and they didn't immediately notice), so its possible rebels could have gotten one.
Rebels are denying involvement through Russian channels and saying they have no surface-to-air capabilities, but apparently this is in the same general area that two Ukranian military planes were shot down and a third fired on.
Until we know more, basically, anyone in the area could have done this, either intentionally or unintentionally as happened with Iran Air 655. My personal belief is that it was rebels using either Russian or captured equipment, and I doubt it was intentional.